On second thought, let me be more explicit. You did write this much about the speciality of workday peak power:

working-day peaks, when power companies have to throw in their most expensive, rarely used resources

However, the situation when power companies have to throw in their most expensive, rarely used resources is not average peak load, nor average weekday peak load, but peak loads higher than that. Such peaks can appear at any time in the week when caused by a baseload plant shutdown. With the spread of intermittent renewables, these peaks can appear at any time of the day or week. Hence my earlier contention that using the standard deviation would be a good measure of price extremes. With that background, I can imagine two possibilities: either I'm not aware of something special about the usefulness of the weekday measure, or that usefulness is dated (reflecting the pre-renewables, pre-deregulation structure of the power market).

_{*Lunatic*, n.
One whose delusions are out of fashion.}